1. Field of the Invention
The present invention relates in general to telecommunications methods and apparatuses and, in particular, pertains to the delivery of answer supervision information from a central office connected to a called party's telephone, through a pulse code modulated carrier to a channel unit coupled to a calling party's telephone equipment for automatic notification to the calling party that the called party has answered.
2. Statement of the Problem
Telephone customers such as hotels and office buildings typically install public branch exchanges (PBX) which allow a number of telephones to access the telephone network through a single telephone network channel. It is important to these users that each telephone served by the PBX is charged accurately for the time the telephone is used, especially for toll connections. Ideally, telephone charges should begin as soon as a connection is made. Therefore, it is important to know precisely when the called party answers their phone. The ability to provide information about when a called party has answered a call is referred to as "answer supervision".
Normally, the Central Office (CO) of a local telephone company detects when the called party lifts the hand-set creating an off-hook condition which completes an electric loop from the Central Office. The telephone company starts billing the call from the moment it detects the loop completion. The answer supervision information is passed on to inter-exchange carriers, i.e., the American Telephone and Telegraph Company (AT&T), U.S. Sprint, MCI. Unfortunately, conventional business telephone service does not ordinarily include access to the answer supervision signal.
In the past, all telephone communications occurred through simple wires known as "metallic" interconnections that carried both AC and DC information. Metallic interconnections carry both voice information in the form of electric waveforms, as well as some signal data in the form of direct current signals. An important example of a DC signal is a "reverse battery" signal generated by a Central Office to indicate when a telephone is answered. A reverse battery signal is generated by applying battery voltage (-48 V) to a telephone Tip lead which is normally coupled to ground, and applying ground to the telephone Ring lead which is normally coupled to battery voltage. Answer supervision is provided by the reverse battery signal, and allows telephone companies to accurately charge customers for phone calls upon detection.
Historically, only telephone companies needed answer supervision information since they were the only entity to bill customers for telephone service. Thus, there was little need to make the answer supervision signal available to customers and their telephone equipment. Recently, however, owners of hotel and office building PBXs have desired improved ability to monitor connection time so they can charge individual users accurately. In response, some newer Central Office equipment is capable of sending a reverse battery answer supervision signal to a calling PBX. An example of a Central Office switch circuit capable of providing the reverse battery signal is a DMS100 which is currently supported by software to provide reverse battery answer supervision. Another switch, a 5ESS includes reverse battery circuitry, but is not currently supported by software to provide answer supervision. An older and more common switch is a 1A, which does not even contain the reverse battery circuitry, and so cannot provide answer supervision. Even when this circuitry is included in the Central Office, however, special software drivers are required to provide answer supervision to the PBX. Also, this type of answer supervision is available for only a limited range of calls because the connection from the calling PBX to the Central Office must be made entirely on a metallic network.
FIG. 1 (prior art) illustrates a typical network for providing answer supervision to customer provided equipment (CPE) indicated by dashed box 11. Customer provided equipment 11 includes calling telephone 12 coupled to metallic network 11 through private branch exchange (PBX) 13. Metallic network 11 is illustrated as a two wire interconnect including a Tip lead (T) and a Ring lead (R) commonly used to couple private telephone equipment with a public telephone network. PBX 13 may also be a toll switch.
Using a ground start protocol, when a call originates from CPE 11, PBX 13 connects the Ring lead (R) to a ground potential. Central Office (CO) 16 senses the current on the Ring lead, and acknowledges the connect request by placing a ground potential on the Tip lead of PBX 13. PBX 13 senses the tip ground, makes the loop switch, and removes the ground connection to the Ring lead. At this point, PBX 13 receives a dial tone, and issues pulse dialing or Dual Tone MultiFrequency (DTMF) dialing instructions over metallic network 14.
CO switch 16 attempts to make a connection to called telephone equipment 17 through the CO switch 25 which serves called party's telephone equipment 17. If called telephone 17 is off-hook, and therefore busy, a busy signal will be returned by the CO switch 25 to the calling PBX 13. On the other hand, if the called telephone equipment 17 is not busy, CO switch 25 applies a ringing voltage to the Ring lead (R) of called telephone equipment 17. The ringing voltage is typically a 20 Hz signal having a cadence of two seconds on, four seconds off. The ringing voltage causes called telephone 17 to ring. At the same time, an audible ring back is transmitted by the called party's CO switch 25 to PBX 13 so that the calling party is aware that an attempt is being made to alert the called party of an incoming call.
The preceding steps can be accomplished on both metallic networks and over PCM carriers. The ground-start signalling information is transferred across the PCM carrier using A/B signalling, also called "robbed bit" signalling. U.S. Pat. No. 4,636,584 sets forth a signaling protocol for a channel unit for converting the A and B signaling bits to high and low logic signals.
When called telephone 17 is answered by lifting the handset off-hook, the off-hook condition is detected by CO switch 25. CO switch 25 responds to the off-hook condition by removing the ringing voltage. CO switch 25 transmits a 37 common channel" signal indicating that called telephone 17 has answered over interoffice pipeline 15 to CO switch 16. Common channel signalling uses a dedicated data transmission line separate from the message data line to transmit signal and control data between CO switch 16 and CO switch 25. Where Central Office 16 is a typical Central Office switch, it is not capable of returning any kind of indication to PBX 13 that called telephone 17 has answered. If, on the other hand, Central Office 16 includes circuitry for providing reverse battery signalling, and Central Office 16 includes the necessary software, Central Office 16 generates a reverse battery signal by applying battery to the Tip lead and ground to the Ring lead of PBX 13.
PBX 13 uses the answer supervision signal to provide additional services to the telephone user, such as to activate billing equipment 15. It should be understood that in the prior art answer supervision circuit illustrated in FIG. 1, the reverse battery signal can only be transferred by Central Office 16 over a short distance metallic interconnection 16. Thus, if Central Office 16 were separated from PBX 13 by any kind of digital transmission line, the reverse battery signal could not be provided to PBX 13. Until now, only customers that were fortunate enough to be located geographically close to a CO switch that provide reverse battery answer supervision could use the reverse battery signal. Customer's that were served by a typical CO switch without reverse battery capability simply could not obtain the benefits of receiving an answer supervision signal. A need exists to provide a reverse battery answer supervision signal to customers who are geographically distanced from a CO switch that provides reverse battery answer supervision signals.
Telephone companies continue to replace large parts of the metallic interconnect network with digital carrier networks. Digital carrier networks involve processing an analog voice signal by circuitry that converts the analog signal into a digital signal and combining the digital signals from many sources by time division multiplexing (TDM). The circuitry that performs the conversion and TDM is called a channel unit. Many channel units are combined, together with interface circuitry, to form a "channel bank" located at a remote Central Office (shown in FIG. 2). The channel bank transmits the multiplexed digital signal on a pulse code modulated (PCM) carrier such as a T-carrier transmission line. AT & T provides the D4 digital channel bank family commercially to operating companies and other telephone companies. The characteristics of the D4 digital channel bank family are fully discussed in the November, 1982, Volume 61, No. 9, Part 3, issue of the Bell System Technical Journal entitled "The D4 Digital Channel Bank Family."
One problem with PCM carrier systems is that signal information is not always transmitted between the Central Office and the calling customer's equipment. In particular, the reverse battery signal traditionally used for call supervision can only be transmitted on metallic networks because it is a direct current signal. Until now, no provisions have been made to communicate reverse battery information across a PCM carrier on a ground start FX line. Because the originating telephone equipment did not have access to the answer supervision signal, telephony designers have provided answer supervision by 1) voice detection 2) ring cadence detection, and 3) timing after the last digit dialed. Each of these methods enjoy some success, but require complex logic and remain plagued by false answer detection or no detection when a call is in fact answered. These errors result in erroneous charges or no charges at all, and are a consequence of inexact answer supervision.
U.S. Pat. No. 4,916,733 issued to Smith et al in April, 1990 shows an apparatus which generates an answer supervision signal by measuring the timing cadence of tones received by the calling telephone equipment. Because the calling equipment will usually receive tones having a regular cadence, such as a ring back signal, answering can be detected by an abrupt change in the cadence of the received tones. However, this system depends on regular cadence provided by the U.S. telephone network, and fails to detect answering when the called telephone equipment is in another country.
U.S. Pat. No. 4,926,469 issued to Smith et al on May, 1990 discloses an apparatus for providing answer supervision to privately owned pay telephone equipment. This apparatus includes a microprocessor for analyzing tones received by the calling telephone while waiting for the called telephone to answer. This apparatus improves upon the '733 apparatus by providing more complex tone analysis to distinguish between an answered call, and noise caused by signals such as busy signal.
U.S. Pat. No. 5,134,651 issued to Ortiz et al on Jul. 28, 1992 discloses a method an apparatus for providing answer supervision by transmitting a 2,100 Hz. tone towards the called telephone, and monitoring the receive line of the calling telephone equipment. Because the audible tone is received back into the calling telephone equipment only when the circuit between the Central Office and the called telephone is completed, the returned tone indicates the called telephone equipment has answered. Nevertheless, the '651 patent provides answer supervision by generating a derived answer supervision signal, and does not provide access to an answer supervision signal generated by the Central Office serving the called telephone equipment.
A simple and common method for providing answer supervision is to measure elapsed time from the last digit dialed, and to assume that the called telephone has answered after a predetermined time interval. Typically a forty-five second time interval is used, which allows about seven to eight rings before the called party is assumed to have answered. Because billing is started only after the predetermined time interval, any call that answers in less than the predetermined time is billed less than it should. In fact, if a call is completed before the predetermined time, no billing would occur, even though the owner of the PBX would be billed by the telephone company (which has answer supervision for internal use) for the connection. Similarly, a persistent caller who waits for more than about seven to eight rings for an answer will be charged as if a connection were made even if the called party never answers. This results in customer dissatisfaction.
A need exists for an apparatus that sends answer supervision information generated by a Central Office, such as local Central Office 16 to a customer, most notably a hotel or motel with their own PBX such as PBX 13, to allow the customer to provide accurate billing that commences with the return of the answer supervision information. Likewise, a need exists for an apparatus that provides the answer supervision signal, such as a reverse battery signal, in a format that is compatible with ground start operation when the customer's equipment is connected to the Central Office through a foreign exchange channel. A need further exists to obtain this answer supervision without relying on circuitry that detects an answer by analyzing tones, waveforms, and noise on a transmission line to detect when a call is completed.